Neuronal nitric oxide synthase (nNOS) catalyzes the oxidation of L-arginine to L-citrulline in the central nervous system, generating nitric oxide (NO), a critical neurotransmitter. Significant research has implicated the overexpression of nNOS— and overproduction of NO—in various neurological diseases, including Parkinson's, Alzheimer's, and Huntington's diseases, as well as neuronal damage due to stroke. Inhibiting endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) is, however, undesirable, because these isozymes are responsible for maintaining crucial body function. Thus, selective inhibition of nNOS over its closely related isoforms, eNOS and iNOS, can provide a promising strategy in developing therapeutics for the treatment of neurodegenerative diseases.
Through on-going research of nNOS selective inhibitors, a pyrrolidine-based compound with an enthanamino linker moiety (cpd JI10, discussed below) was found to provide great potency (Ki=15 nM) and very high selectivity for nNOS over eNOS (2100 fold) and iNOS (630 fold). However, despite the promising inhibitory activity of this compound, further application to neurodegenerative therapeutics has been impeded by several structural characteristics. First, the flexible m-fluorophenyl ethanamino tail brought multiple rotatable bonds to the inhibitor, limits its potency and selectivity. In addition, the benzylic position of the m-fluorophenyl ring is highly susceptible to metabolic oxidation reactions. More importantly, positive charge at physiological pH, relating to the two amino groups—including one group in the aforementioned ethanamino linker, decreases the chance of such a compound to penetrate the blood brain barrier (BBB).